Semiconductor pressure sensor with slanted resistors

Abstract

A diaphragm is formed in a silicon chip by etching a rectangular cavity in one side thereof and piezoresistive resistors are formed in the other surface of the chip to sense stress changes on the diaphragm due to pressure changes. At least one resistor is placed along the edge of the diaphragm where a sharp stress peak occurs. To avoid the problem of inaccurate placement of the resistor relative to the peak, the resistor is slanted with respect to the stress ridge at a small angle of 10° to 20°. This makes the resistor placement and cavity alignment much less critical thereby assuring greater uniformity of response from one sensor to another at the expense of signal size for a given pressure change on the device.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 
     
       1. A pressure sensing element comprising a semiconductor chip containing a cavity defining a diaphragm subject to deflection in response to pressure, the diaphragm having an elongated high strain zone adjacent and parallel to the diaphragm edge wherein upon diaphragm deflection a localized high strain peak is produced in the high strain zone, and   a plurality of piezoresistive resistors in the chip for sensing strain in the diaphragm, at least one of said resistors for measuring the strain in the strain zone, said one resistor being elongated and slanted across the high strain zone with the longitudinal resistor axis disposed at an angle in the approximate range of 10° to 20° with respect to the said zone, so that small variations in resistor placement relative to the strain zone result in only minor variations in resistor response to diaphragm strain.   
     
     
       2. A pressure sensing element comprising a semiconductor chip containing a cavity defining a diaphragm subject to deflection in response to pressure, the diaphragm, upon deflection, having an elongated high strain ridge adjacent and parallel to the diaphragm edge, said   a plurality of piezoresistive resistors in the chip for sensing strain in the diaphragm, at least one of said resistors for measuring the strain in the strain ridge, said one resistor being elongated and slanted across the high strain ridge with the longitudinal resistor axis disposed at an angle in the approximate range of 10° to 20° with respect to the said ridge, so that small variations in resistor placement relative to the strain ridge result in only minor variations in resistor response to diaphragm strain.   
     
     
       3. A pressure sensing element comprising a semiconductor chip containing a cavity defining a diaphragm subject to deflection in response to pressure, the diaphragm, upon deflection, having an elongated high strain ridge adjacent and parallel to the diaphragm edge wherein due to manufacturing tolerances the strain ridge occurs anywhere within a predetermined tolerance band defined by limits parallel to the diaphragm edge, and   a plurality of piezoresistive resistors in the chip for sensing strain in the diaphragm, at least one of said resistors for measuring the strain in the strain ridge, said one resistor being elongated and slanted across the high strain ridge and traversing the tolerance band with both ends of the resistor disposed just outside the limits so that small variations in resistor placement relative to the strain ridge result in only minor variations in resistor response to diaphragm strain.   
     
     
       4. A pressure sensing element comprising a semiconductor chip containing a cavity defining a diaphragm subject to deflection in response to pressure, the diaphragm, upon deflection, having an elongated high strain ridge adjacent and parallel to the diaphragm edge, and   a plurality of piezoresistive resistors in the chip for sensing strain in the diaphragm, at least one of said resistors for measuring the strain in the strain ridge, said one resistor being elongated and slanted across the high strain ridge with the longitudinal resistor axis disposed at an angle to the strain ridge large enough to significantly increase yields during manufacture thereof compared to resistors parallel to the strain ridge and small enough to have good resistor response to diaphragm strain, so that small variations in resistor placement relative to the strain ridge result in only minor variations in resistor response to diaphragm strain.   
     
     
       5. A pressure sensing element comprising: a semiconductor chip containing a cavity defining a diaphragm subject to deflection in response to pressure, the diaphragm, upon deflection, having an elongated high strain ridge adjacent and parallel to the diaphragm edge wherein due to manufacturing tolerances the strain ridge occurs anywhere within a predetermined tolerance band defined by limits parallel to the diaphragm edge, and   an elongated piezoresistive resistor in said chip effective to measure strain in the strain ridge, the resistor lying across the tolerance band with the resistor ends outboard of the tolerance band and adjacent the limits thereof, and a direction of elongation slanted in relation to the tolerance band and as nearly parallel to the diaphragm edge as the tolerance band permits, whereby small variations in resistor placement relative to the strain ridge result in only minor variations in resistor response to diaphragm strains.   
     
     
       6. A pressure sensing element comprising: a semiconductor chip containing a cavity defining a diaphragm subject to deflection in response to pressure, the diaphragm, upon deflection, having an elongated high strain ridge adjacent and parallel to the diaphragm edge wherein due to manufacturing tolerances the strain ridge occurs anywhere within a predetermined tolerance band defined by limits parallel to the diaphragm edge, the chip comprising a semiconductor material of one conductivity type, and   two pairs of elongated piezoresistive resistors in said chip effective to measure strain in the diaphragm, each resistor including an elongated area of a conductivity type opposite to the conductivity type of the diaphragm, one pair of resistors having a direction of elongation generally perpendicular to the tolerance band, the other pair of resistors each lying across the tolerance band and having its direction of elongation slanted in relation to the tolerance band with its ends just outboard of the tolerance band so that small variations in resistor placement relative to the strain ridge result in only minor variations in resistor response to diaphragm strain.   
     
     
       7. A pressure sensing element comprising: a semiconductor chip having a cavity in one face thereof having straight edges which defines a substantially rectangular diaphragm subject to deflection in response to pressure, the diaphragm, upon deflection, having elongated strain ridges adjacent and parallel to the straight edges of the diaphragm,   a first pair of elongated piezoresistive resistors in an opposite face of said chip, each extending across respective first opposed edges of said diaphragm and located substantially perpendicular to said first edges,   and a second pair of elongated piezoresistive resistors in an opposite face of said chip, each of said second resistors being slanted across a high strain ridge located adjacent respective second opposed edges of said diaphragm, the longitudinal resistor axis of said second resistors disposed at an angle to a respective strain ridge that is large enough to increase yields during manufacture thereof compared to resistors parallel to the strain ridge and small enough to have good resistor response to diaphragm strain, so that small variations in resistor placement relative to location of the cavity edges result in only minor variations in resistor response to diaphragm strain.